In order to stabilize behavior or a posture of a vehicle during turning, according to the conventional turning performance improving control, driving force and braking force of the vehicle are automatically controlled in conjunction with a steering operation performed by a driver. For example, Japanese Patent Laid-Open No. 2005-256636 describes a vehicle stability control system for controlling a driving force in a manner such that a stability factor of the vehicle follows a target value thereof. According to the teachings of Japanese Patent Laid-Open No. 2005-256636, specifically, a basic required driving force during turning is calculated as a physical amount to be generated by drive wheels, and a target value of a stability factor is calculated based on estimated loads applied to the front and rear wheels and an estimated turning radius of the vehicle. The physical amount calculated as the basic required driving force is corrected in a manner such that an actual stability factor of the vehicle follows the calculated target value, and the driving force corresponding to the physical amount thus corrected is generated by the drive wheels.
Japanese Patent Laid-Open No. 2011-218953 describes a driving force controlling system for controlling a driving force of drive wheels in a manner such that a steering characteristic of a vehicle follows a target steering characteristic.
As described, the vehicle stability control system taught by Japanese Patent Laid-Open No. 2005-256636 is configured to control the driving force in a manner to achieve the target stability factor of the vehicle. On the other hand, the driving force controlling system taught by Japanese Patent Laid-Open No. 2011-218953 is configured to control the driving force in a manner to achieve the target steering characteristic. That is, the control systems taught by those prior art documents are configured to carry out a turning performance improving control for stabilizing the vehicle behavior during turning.
Under the turning performance improving control, the driving force is controlled automatically in accordance with a running condition and a behavior of the vehicle irrespective of the driving force required by operating an accelerator or a brake. To this end, for example, a target yaw rate of the vehicle is calculated during turning based on a vehicle speed and a steering angle, and a driving force and a braking force of the vehicle are controlled automatically in a manner to achieve the target yaw rate of the vehicle.
Basically, a vehicle is provided with a transmission for transmitting an engine torque to the drive wheels in accordance with a speed ratio. In the conventional art, for example, an automatic transmission such as a geared transmission having a planetary gear unit and a belt-driven continuously variable transmission are used in vehicles. A gear stage or a speed ratio of the automatic transmission is altered in accordance with a driving force required by operating an accelerator or a brake.
However, if the conventional turning performance improving control is executed in the vehicle having the automatic transmission, a speed change control of the transmission and the turning performance improving control interfere with each other, and consequently a speed change operation is carried out unintentionally. For example, the driver would unconsciously operate the accelerator or brake to adjust the driving force changed by the turning performance improving control. Consequently, the speed change operation would be carried out unnecessarily.
Given that the geared transmission is used in the vehicle, the speed change operation is carried out with reference to a speed change map or a shifting diagram shown in FIG. 6. Basically, the speed change map determines a shifting line based on a vehicle speed and an engine load such as an operating amount of the accelerator or an opening degree of a throttle valve. According to the example shown in FIG. 6, an upshift line and a downshift line are determined based on a vehicle speed and an opening degree of accelerator. That is, the speed change operation is executed upon satisfaction of an execution condition. Specifically, the speed change operation is executed when an operating point representing a running condition of the vehicle is changed across the shifting line. For example, when such operating point moves across the upshift line drawn between the first stage and the second stage, an upshifting is carried out to shift the gear stage from the first stage to the second stage. By contrast, when such operating point moves across the downshift line drawn between the third stage and the second stage, a downshifting is carried out to shift the gear stage from the third stage to the second stage.
However, under the speed change control using the shifting map, the speed change operation would be carried out unintentionally irrespective of the actual running condition as shown in FIG. 7. For example, if the turning performance improving control is carried out when the vehicle is driven at the operating point O0 in FIG. 7, the driving force will be reduced by the turning performance improving control and the operating point is shifted to A0. Thus, the driving force is automatically reduced in spite of keeping the opening degree of the accelerator constant. Consequently, the vehicle is driven at the operating point (A0 in FIG. 7) at which the driving force is reduced.
If the driving force is thus reduced automatically as a result of carrying out the turning performance improving control, the driver would depress an accelerator pedal to compensate such reduction in the driving force. Consequently, an opening degree of the accelerator is increased and the operating point shown in FIG. 7 is thereby shifted from the point O0 to the point BO across the shifting line. In this case, specifically, the operating point is shifted across the downshift line so that the down shifting of the transmission is carried out. Thus, the downshifting would be carried out unintentionally even if the vehicle should be driven under the gear stage to be established at the point O0.
Therefore, the conventional turning performance improving control is required be improved so as to properly control the driving force of the vehicle having the automatic transmission without causing an interference with the speed change control of the transmission.